It's usually some sort of shitty software at play. Hell, at this point Hulu only works half the time and usually freezes crashes of course during transitions to ads because they're greedy fucks. Trying to watch anything online that uses Adobe anything? Good luck. Open a tab, click on the video which opens another tab, then click on 'full screen' which opens another window and you can't close the previous tab. Add in freezing up anytime you switch between full screen and/or any ads that start to play. Sucking way more resources than needed (I swear it actually starts a second stream for fullscreen). Makes watching anything online such a chore you wonder if the cable companies aren't paying Adobe to be intentionally shitty.
Unfortunately it often seems to be the combo of Firefox and the other software these days, and I often find myself just using chrome. Hell, even using IE at times is a nightmare navigating but will stream with no problems.